Remove ff from filtered search for iterations

What does this MR do and why?

We had customers reporting not being able to see Iteration in the filtered search dropdown because, although we had the issue_boards_filtered_search ff enabled globally, the iteration_cadences ff was off for a majority of our customers.

This MR removes the guard for Iteration token.

Notes

It seems like even with iteration_cadences off, we are still able to get iterationCadences key from graphql.

Repro

• Feature.enable(:issue_board_filtered_search)
• make sure that :iteration_cadences is off.
• Go to Project/Group Issue Board and make sure you can filter by Iteration

Resolves #350123 (closed)

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